How can I parent a rig to another rig?
When animating a character they may need to pick up objects and move them around. The ChildOf constraint is useful for this. The advantage of using the ChildOf constraint is that you can animate the influence to allow the character to pick up and put down the object.

As you can specify an armature and bone to parent to you can also parent different object bones to different character bones, allowing you to have the character hold a piece in each hand and stretch them, or hold the doll body in one hand and lift the doll arm in the other.